In multi-color printing systems, a limited number of color separations are used for marking a substrate for achieving a wider variety of colors, with each separation marking the substrate using discrete shapes, such as dots having a circular or oval shape, or periodic line patterns. This concept is generally known as color halftoning, and involves combining two or more patterned separations on the substrate. The selection of color separations and halftone design are carefully chosen for achieving a visual effect of the desired color.
Many prior art printing systems use cyan, magenta, yellow and black (also referred to as CMYK) color separations that mark a substrate using discrete cluster dots. In accordance with one prior art method, the dots may be marked in a dot-on-dot fashion, by marking the substrate with a first and second color separation, with the dots of the second color separation superimposed over the dots of the first color separation for achieving the desired color. In accordance with a second prior art method, the dots are applied in a dot-off-dot fashion, with the dots of the second color separation placed in the voids of the dots the first color separation for achieving the desired color. Multi-color printing systems are susceptible to misregistration between color separations due to a variety of mechanical related issues. For both dot-on-dot and dot-off-dot rendering, color separation misregistration may cause a significant color shift in the actual printed color that is noticeable to the human eye.
Another marking method of rotated cluster dot sets is widely used since anomalies (e.g., color shifts) due to color separation misregistrations are subtle and less detectable by the human eye. However, even in these cases, color misregistrations may be objectionable, particularly at edges of objects that contain more than one separation. Therefore, it is important to characterize color separation misregistration in order to perform corrective action in the print engine.
Many prior art methods for characterizing misregistration of color separations include using physical registration marks. The registration marks include two fine straight lines, each line formed using a different color separation. The two lines are aligned and joined to form one straight line. Alignment of the two lines is analyzed, with misalignment indicating misregistration of one of the color separations relative to the other. The analysis may include studying the printed registration marks with a microscope and visually determining if misregistration has occurred. Such analysis is tedious and not conducive to automation. The analysis may include imaging the marker with a high resolution scanning device and analyzing the high resolution scanned image using complex software for determining the positions of the registration marks relative to one another. This type of analysis may require expensive high resolution scanning equipment and may involve a significant amount of computational power. Furthermore, this type of processing is prone to error due to even small defects, such as toner splatter.
In another prior art method used for higher end printer devices outputting high volume and/or high quality images, misregistration of color separations is characterized by measuring the transition time between the edges of two primary separation patches (e.g., cyan and magenta) on a moving photoreceptor belt. The patches have angled edges (e.g., chevrons) that allow the determination of misregistration in both the fast scan direction (transverse to the longitudinal axis of the photoreceptor belt) and slow scan direction (parallel to the longitudinal axis of the photoreceptor belt). Simple photo-detectors are used to measure the time between the moving edges of the chevrons, and this may in turn be used to compute the misregistration in both slow and fast scan directions. A drawback to this method is the inability to take misregistration measurements across the page, as the photodetectors are present in only two or three locations across the width of the photoreceptor belt. This method of separation misregistration characterization has been designated for use with high end printer systems due to the cost of the photodetectors and associated analysis software.
Other prior art methods include, as mentioned above, using periodic line patterns for detecting color misregistration. For example, FIG. 1 shows a prior art color registration line pattern used to detect color registration. In the color registration pattern identified by reference numeral 100, black is the reference color. The 7's (element 110) are used to determine lateral misregistration and the dashes (element 120) are used to determine process direction misregistration. Even though not evident from the black and white drawing shown by FIG. 1, the 7's alternate between a colored 7 and a black 7. Each color/black pair of 7's gives one set of lateral measurements. In FIG. 1, there are three lateral measurements per color for a total of 18 marks, and nine process direction measurements per color for a total of 36 marks. The process direction is shown by arrow 130.
FIG. 2 illustrates an enlarged view of FIG. 1. The pattern is identified by reference numeral 200. All dimensions in FIG. 2, for illustrative purposes, are in 1200 dpi. FIG. 2 also shows the position of a mark-on-belt (MOB) sensor 250 and the MOB sensor's line of sight 260. The pattern includes a segment or mark 16 for each color (i.e., cyan 210, magenta 220, yellow 230, and black 240). The line pattern is repeated nine times for each color for a total printout of 36 line segments or marks. Only the number of line segments or marks is counted by the misregistration system. Thus, if there is an error, the color associated with the error is not known.
As each segment passes under the view of the MOB sensor 250, a square wave 300 is generated for each segment as shown by FIG. 3. All the rectangles in the square wave 300 have the same width, since all segments as shown by FIG. 2 have the same width. Reference numerals 310 and 320 respectively identify the time at which the leading edge of the cyan segment is detected by the MOB sensor 250 and the time at which the trailing edge of the black segment is detected by the MOB sensor 250. The center of each segment is shown by reference numeral 330.
In order to detect color-to-color misregistration using the pattern 200 shown by FIG. 2, a time stamp is issued by a processor or controller for the leading edge and trailing edge of each segment. These two time stamps are averaged by the processor to calculate a time stamp for the center of each segment or mark. The time between the center of each color segment (CMY) and the center of the reference segment (K), along with knowledge of the process speed, permits the processor to calculate the position of each color segment relative to the reference segment and the position of the reference segment relative to the MOB sensors. These values are compared by the processor with the nominal positions of each segment to generate a positional error (in the process direction) for each CMYK pattern.
The MOB sensors have no concept of which color segments or marks they are actually determining because they see all the colors almost equally. In order to determine which colors are being read, the number of colors detected has to be exactly equal to the number of colors expected. Since the order in which the color segments or marks are laid down is known ahead of time, if the correct number of segments is seen by the MOB sensors, it is assumed that the order of the colors determined corresponds directly to what is expected.
However, if there are too many or too few segments, the color registration controller has no means to determine which colors are missing or which ones are being erroneously detected. Without that information, the processor discards all of the data and does not issue a correction command to the color registration actuators. The result is either a productivity loss (if an additional color registration iteration needs to be performed), or a performance issue (if the controller chooses not to make another attempt and the current levels of misregistration remain unchanged).